1. Field of the Invention
The present invention relates to filtration devices, vessels and related methods, and more particularly relates to rigid porous filtration devices, vessels and related methods.
2. Description of Related Art
Various filtration systems exist for removing particulates from liquid and gaseous mediums. For example, porous rigid filtration systems, such as ceramic candle filter systems, have been previously developed for use in particulate removal from hot high pressure gases. A filtration system previously disclosed is set out in Backovchin et al. U.S. Pat. No. 5,433,771 issued Jul. 18, 1995, entitled "Hot Gas Filtration System Fail-Safe and Thermal Regeneration Device," which is incorporated herein by reference.
Many processes exist wherein a hot gaseous medium is produced which contains particulate material that must be separated from the gaseous medium, either to prevent pollution or to remove hazardous material or to protect downstream equipment. High temperature filtration of particulate has become an important component in many emerging technologies. For example, advanced coal conversion technologies, such as fluid bed gasification and combustion, are dependent upon the successful removal of particulates at temperatures in the range of about 500.degree. to 1100.degree. C. Other applications which benefit from high temperature filtration range from gas cleaning for biomass gasification to power generation from the incineration of municipal solid wastes. These applications require the removal of particulates from gas streams at high temperature so that process equipment, such as rotating machinery and heat exchange surfaces, remain functional and efficient throughout the use of such equipment.
Rigid ceramic filters are currently being developed and used for separating entrained particles, such as flyash or char, from the hot gases produced in energy generating systems and industrial processes. In these hot gas filtration systems, the ceramic filter often serves as the only filter device for trapping undesirable particles contained in the flow of hot gases which pass through the filtration system.
One type of ceramic filter element, the cross flow filter, is described in Ciliberti U.S. Pat. No. 4,343,631, which is incorporated herein by reference. The cross flow filter comprises several layers of porous ceramic membranes joined together in such a manner as to increase filter area per unit volume. Particle laden gases pass into dirty side channels of the filter and then through the filter membranes where the particles are deposited as cake on the surface of the membranes within the dirty side channels. The cleaned gases then pass into and through the clean side channels of the filter and subsequently exit the filter. Generally within the filter system, a plurality of such filter elements are connected to a single plenum pipe through which the filtered clean gas passes after flowing through the filter elements.
Typically, the filter elements are periodically cleaned by providing a pulse of high pressure gas which is pumped through a pulse cleaning pipe in flow communication with the plenum pipe. The pulse of high pressure gas causes reverse flow through the filter elements which dislodges the cake of particles trapped by the filter elements such that at least some of the cake falls out of the filter elements and is collected and disposed of through a discharge point in a known manner. The high pressure gas used to clean the filters is usually cold due to the known difficulties of pulsing high temperature gas with existing valves.
In addition to the cross-flow filter systems, another popular type of rigid ceramic filter (as briefly mentioned above) is referred to as a candle filter and comprises a hollow cylinder which is closed at one end and flanged at the other for attachment to a tubesheet or blowback plenum, into which cleaned gas passes during the filtration cycle. Particle laden gas passes through the filter elements such that the particles are trapped on the filter surface and the clean gas flows into the hollow center of the candle and out through the open flanged end. In such candle filtration systems, a plurality of candle filters are connected to a plenum pipe such that the clean gas from the plurality of candles flows into the plenum pipe. A pulse of cold gas is periodically blown into the candles for dislodging at least some of the cake from the filter elements.
Candle-type ceramic barrier filters of the general type discussed above are disclosed in U.S. Pat. No. 4,973,458-Newby et al.; U.S. Pat. No. 4,812,149-Griffin et al.; U.S. Pat. No. 4,764,190-Israelson et al.; U.S. Pat. No. 4,735,635-Israelson et al.; and U.S. Pat. No. 4,539,025-Ciliberti et al., each of which is incorporated herein by reference.
While the filtration systems have been generally useful, problems have been recognized in the use of such filtration systems. Since these types of filter devices comprise porous ceramic materials which are subjected to high temperature corrosive environments and to fluctuations in temperature, one or more of the individual filter elements in the system can break. Moreover, since the pulse of high pressure cleaning gas is cold, the elements are subjected to severe changes in temperature such that the filter elements are further prone to breaking. Where one or more of the filter elements in the system breaks, an open path through the filter vessel is available such that the flow of gas through the hole is limited only by the relatively small flow resistance of the orifice left by the missing pieces of ceramic material. Thus, dirty particles remain in the gas after passing through the filtration system, resulting in a substantial decrease in effectiveness of the overall system, as well as, possible adverse effects on the environment and downstream equipment.
For many years, the U.S. Department of Energy has fostered the commercialization of advanced coal-conversion power systems as an efficient means of utilizing the nation's abundant coal supply. High temperature filtration devices are an integral link in the commercialization of these advanced power systems. As set out above, among all the high temperature filtration devices, ceramic candle filter systems are the conventional means of removing fine particulates from high temperature gas streams. Problems with these conventional candle filter devices typically include:
a) A large number of filter elements are needed for a commercial scale system, for example, 384 candles have been used for a 10-MWe pressurized fluidized bed combustion system, and a 100-MWe scale may require as many as 3840 filter elements; PA1 b) The filter vessel required to accommodate thousands of candle elements would necessarily be very large, and would be a factor of the length of the candle filter element, for example, a 10-MWe pressurized fluidized bed combustion system has adopted a three-tier arrangement of filters with most of the space taken by nonfiltering components, such as plenums, ash sheds, and plenum support conduits; and PA1 c) Due to the long slender design and the brittle nature of ceramic candle filter elements, they are generally not resilient to mechanical loads that may induce excessive tensile stresses in the ceramic material. In addition to the high aspect ratio for candle filter elements, the ceramic elements are usually mounted such that the body of the filter is preloaded in tension rather than compression.
Consequently, there is a need and a desire for a porous rigid filtration system which solves one or more of the above problems.